This invention relates to selective coatings for utilization of solar energy. More particularly, this invention relates to a selective black coating for aluminum.
The utilization of solar energy by heat absorption is well known. However, the conversion requires particular materials or systems which are not readily available or which are too expensive to be of practical value. One such system relies on lenses to concentrate the solar rays using parabolic reflectors or the like. In another system which has achieved some limited experimental success, solar collector panels are coated with materials which will absorb the solar radiation. Typically, such materials are metal sheets painted with a flat black paint which will absorb the solar radiation and conduct (via the metal) the heat to appropriate conversion means which may, for example, comprise the heating of water which is then pumped to a utilization point. While it is conventionally known that blackened surfaces are better absorbers of solar energy, it is also characteristic that such surfaces have a high emissivity. This is particularly undesirable from the standpoint of emission of longer wavelength energy such as infrared wavelengths greater than 6 microns which will cause loss of the absorbed energy.
It is therefore desirable that a coating exhibit a high absorption of solar energy, particularly energy in the visible light and near infrared wavelengths (from 0.38 to 1.9 microns) which comprise a majority of the solar spectrum. It should also be characterized by a low emissivity particularly in the area of longer (infrared) rays, i.e. greater than 6 microns.